Because of lack of oxygen , low atmospheric pressure and increased altitude . Due to the sudden changes in the atmosphere , our blood vessels get burst because of pressure of blood increased. 
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Due to low atmospheric pressure, our blood vessels rupture &it occurs bleeding
this is a very common thing which happens with almost all the mountaineers.we are are habituated in the place we ion.the sudden change in the amount of oxygen or the sudden decrease in the amount of oxygen bursts our blood vessels and this leads to nose bleeding.
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